An acronym (Same Lingual Opposite Buccal) describing a parallax radiographic technique used to identify the position of ectopic teeth (usually maxillary canines). It compares the object movement with the x-ray tube head movement. If the tube head moves mesially, the image of a lingual object will also move mesially on the film, i.e. in the same direction. The image of a buccal object will move in the opposite direction. The movement of the lingual or buccal object is compared to objects whose locations are known; this will usually be an erupted tooth.
http://www.manifestinteractive.com/demos/elsevier/haring/SlobRule.html This website provides a graphic illustration of how the rule operates.